With reference to FIG. 1, a ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, a power gearbox 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, an intermediate pressure turbine 18, a low-pressure turbine 19 and a core engine exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first (main) air flow A into the intermediate pressure compressor 14 and a second (bypass) air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the air flow A directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 15, 15, and the fan 12 through the power gearbox 13, by suitable interconnecting shafts.
The power gearbox 13 enables the fan 12 to be operated at a lower rotational speed than the low pressure shaft and thus low pressure turbine 19. This enables the low pressure turbine 19 to operate at a higher rotational speed thus requiring fewer stages, and so increasing efficiency and reducing weight.
The rotational speed of the fan 12 allows higher bypass ratios, leading to reductions in both fuel consumption and generated noise. A large part of the noise reduction is due to the reduced fan tip speeds.
The high power requirement of the propulsive fan 12 and the packaging constraint leads to an epicyclic gear train that in turn has a large diameter which takes up valuable space in the engine core. The large gearbox diameter also results in significant centrifugal loading on gearbox components, particularly the planet gears.
One known technique for reducing the centrifugal loading on the planet gears is to reduce their axial width. However, reducing the axial width of the planet gear may lead to circumferential distortion, particularly where the planet gear teeth mesh with the sun and ring gear teeth. This circumferential distortion causes a change in the loading on the planet gear bearing, which in turn leads to higher bearing stress and reduced bearing life.